1. Field of the Invention
The present invention relates to a soft-start voltage circuit, and more particularly, to a soft-start voltage circuit for providing a soft-start voltage to a DC/DC converter.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram illustrating an error amplifier EA in a DC/DC converter. The error amplifier EA is utilized to compare a feedback voltage VFB from an output voltage VOUT of the DC/DC converter with a reference voltage VREF or a soft-start voltage VSOFT for generating an error signal SERROR to adjust the level of the output voltage VOUT of the DC/DC converter as desired.
Generally speaking, at the start phase of a DC/DC converter, the output voltage VOUT of the DC/DC converter is still at a very low level. As a result, if the error amplifier EA compares the feedback voltage VFB from the output voltage VOUT with the reference voltage VREF at the time, the gain of the error signal SERROR generated by the error amplifier EA is relatively high. In such condition, the DC/DC converter generates a current with a very large magnitude, which is so called inrush current, for raising the output voltage VOUT to the required voltage level. In this way, the inrush current may affect the voltage level of the input voltage source of the DC/DC converter. Thus, at the start phase of the DC/DC converter, the error amplifier EA compares the feedback voltage VFB from the output voltage VOUT with the soft-start voltage VSOFT. In this way, the gain of the error signal SERROR generated by the error amplifier EA is not too high so as to reduce the inrush current of the DC/DC converter and therefore the voltage level of the input voltage source of DC/DC converter at the start phase of the DC/DC converter is not lowered.
Please refer to FIG. 2. FIG. 2 is a diagram illustrating the relationship between the soft-start voltage and the reference voltage. For the soft-start voltage having the characteristics described above, the soft-start voltage must be a voltage which rises gradually. When the soft-start voltage VSOFT is lower than the reference voltage VREF, which is so called the soft-start phase of the DC/DC converter, the error amplifier EA compares the feedback voltage VFB with the soft-start voltage VSOFT for generating the error signal SERROR. When the soft-start voltage VSOFT is higher than the reference voltage VREF, which is so called the normal phase of the DC/DC converter, the error amplifier EA compares the feedback voltage VFB with the reference voltage VREF for generating error signal SERROR. For having a longer soft-start phase, the slope of the soft-start voltage VSOFT when the soft-start voltage VSOFT rises must be flatter. That is, the more gradually the soft-start voltage VSOFT rises, the longer the start phase is, and the smaller the load current drained from the output voltage source VOUT is. However, for realizing the characteristic of the soft-start voltage VSOFT to rise gradually, generally it is achieved by a capacitor with relative large capacitance or a charge current with relative small magnitude.
Please refer to FIG. 3. FIG. 3 is a diagram illustrating the conventional soft-start voltage circuit 300. As shown in FIG. 3, the soft-start voltage circuit 300 comprises a capacitor CSOFT and a current source ISOFT. The voltage over the capacitor CSOFT is served as the soft-start voltage VSOFT. As described in the previous paragraph, in the soft-start voltage circuit 300, it is required that the capacitance of the capacitor CSOFT is large enough or the current provided by the current source ISOFT is small enough for providing a gradually rising soft-start voltage VSOFT to prolong the start phase. However, the capacitor CSOFT occupies a quite large area in a general chip. Thus, utilizing the capacitor CSOFT with the large capacitance to realize the soft-start voltage circuit 300 wastes a substantial area. Moreover, when the current source ISOFT provides a small enough current, the current source ISOFT is easily affected by the fabrication so that the current provided by the current source ISOFT is not as the same as expected. In other words, the period of the soft-start phase is affected and not as expected. Therefore, regardless of adjusting values of the capacitor CSOFT or the current source ISOFT, it is quite inconvenient for the user.